my chickens and geese found the way to eat tomato leaves. luckily never had any problems. I guess if they ate it on a daily basis that would harm them.
Quantity matters from what I read. Humans, and many other animals, can eat something bad and vomit it out of their systems. Not so with chickens. So, eating plants that are toxic to them can make them sick or kill them before they clear it out of their system. Best to have those plants separated from the chickens.